Abstract
This mineralogical and geochemical study of platinum-group minerals (PGM) and base-metal sulfides (BMS) is focused on the mineralised P1 and the P2 units of the Platreef found in the Northern Limb of the Bushveld Complex at the Akanani Lonmin Project Area. The P2 unit within the Akanani Project Area in previous studies has shown an abundance of Pt-Pd Bismuthotellurides, however, the sample spread was limited only to the P2 unit and the BMS had not been a key focus. In the current study, however, a wider sample set is available which incorporates both the P1 and P2 units making this a more comprehensive study.
The study made use of the Mineral Liberation Analyzer (MLA) to determine the mineralogy of PGM, their grain size distribution as well as their mineral associations in both the P1 and the P2. PGE-bismuthotellurides were found to be the most abundant PGM in both the P1 and P2 unit. The least abundant were PGE-sulfides and electrum. The PGM of the P1 unit were overall coaser grained than those in the P2 unit.
The PGM mineral association in the P1 unit is the same as that of P2 unit. The PGM were strongly associated with primary silicates (this was due to the high modal abudance of these primary silicates and not by preference of the PGM). The PGM were further found to be associated with BMS namely chalcopyrite, pentlandite, pyrrhotite and pyrite, through grain boundary contact, near grain boundary inclusions and fully encapsulated inclusions (this association is thought to reflect the formation of PGM through exsolution from a monosulfide solid solution (mss) and the actual preferred association of PGM with BMS). PGM showed a significant association with secondary silicates (this reflects the role played by hydrothermal fluids in the redistribution of PGM). PGM were least associated with other minerals.
This study further made use of the Electron Microprobe (EMPA) and Scanning Electron Microscope (SEM) to study the chemistry of the BMS (chalcopyrite, pentlandite, pyrrhotite and pyrite). The chemistry of these BMS in the P1 unit showed a high chemical variation more especially for the Fe-Ni-S minerals. In contrast, the BMS of the P2 unit showed a more uniform chemistry. The high chemical variation in the constituents analysed is thought to be due to the alteration of the P1 unit.
M.Sc. (Geology)